Sensitive photocell and circuit



Nov. 5, 1946. c. w. HANsELL 2,410,732

SENSITIVE PHOTOCELL AND CIRCUIT Filed March 8,' 1943 2 sheets-sheet 2 GUIDES/V7' POWER ,Soc/@CE HUD/O e? OUTPUT D. c. ,fol/APCE /A/l/ENTOR.'

Patented Nov. 5, 1946 SENSITIVE PHOTOCELL AND CIRCUIT Clarence W. Hansell, Rocky Point, N. Y., assignor to Radio Corporatonof America, a corporation of Delaware Application March 8, 1943, Serial No. 478,423

(Cl. Z50-41.5)

11 Claims. l

The present invention relates generally to photocell circuits, and more particularly concerns a highly sensitive phototube in the form of a phototube secondary emission amplifier.

One of the objects of the present invention is to provide a simplified circuit for electrically reproducing sound waves which have been recorded on nlm.

Another object is to provide a single unit which serves the dual purpose of a phototube and high gain amplier.

A further object is to provide a magnetron device which is sensitive to light waves and supplies an output current whose magnitude is approximately proportional to, and follows modulations of said light waves.

A still further object is to provide a phototube device arranged in a magnetic eld, whereinV the photoemissive cathode also serves as a secondary electron emitter.

Briefly stated, the invention is based upon my discovery and appreciation of the fact that electrons emitted from a surface may be made to return to the surface with sufficient energy to cause secondary emission and a growth in rate of emission through a combination of electric and magnetic fields, one or both of which is changing in a correct direction at a suciently rapid rate. This phenomenon is discussed in detail in my copending application Serial No. 477,062, led February 25, 1943, to which reference is made. I employ this phenomenon to make asingle surface serve the same function in multiplying photoemissive electrons by means of secondary emission as though a large number of surfaces had been used in cascade.

A simple form of the invention comprises a phototube device employing a photoemissive cathode which is also a good secondary electron emitter, so arranged in a magnetic field that the magnetic ux lines extend parallel to the cathode and at right vangles to the electric eld between an anode and the cathode. The phototube device of the invention is thus built in the manner of a magnetron. A modulated light, such as is produced by light shining through a sound track on a motion picture film, is arranged to shine n the cathode to provide an initial or primary emission. By means of a suitable alternating current potential, at a suitable frequency, applied between the anode and cathode, there is obtained ata particular part in the cycle a condition of rising potential required for the accumulation of space charge due to the photoemission, followed about a quarter cycle later by the conditions required for rapid multiplication of the space charge due to the emission of secondary electrons from the cathode. By suitable adjustments of the alterhating current potential and frequency, which can be accomplished automatically, it is possible to make the growth of secondary emission and space charge such that nearly saturation value is reached on each cycle, just as it is in the case of the low emission magnetron oscillator described in my copending application supra. A modulated direct current, carried by the electron emission from the cathode and supplied from a direct current potential source in circuit with the anode or an associated electron collector, provides the useful output power. This direct current may be enormously greater than the initial photoemission current. It should be understood, however, that in the practice of the present invention it is not desiredto reach the ultimate maximum space charge limited secondary emissive current in each cycle, because such a condition of maximum secondary emission current would produce limiting in the phototube and distortion of the modulation to be reproduced.

A feature of the invention lies in the use of a photo surface which is a secondary emitter for the photo electrons.

One advantage of the dual purpose phototube and amplier of the invention is that it has an extremely large electron multiplying ratio and is simpler to build and easier to operate than prior constructions which employ a multiplicity of electron multiplying stages in series.

A more detailed description of the invention follows in conjunction with the drawings, wherein:

Figs. 1 and la illustrate diiferent cross-sectional views of a 'simple form of phototube secondary emission amplier in accordance With the invention. Fig. la is a view taken along the line Ia-Ia of Fig. l; and

Figs. 2, 3 and 4 schematically illustrate three different complete circuit embodiments of the invention.

In Figs. l and la, the phototube amplifier is shown as including a cylindrical cathode I0 which is treated to be photoemissive and also capable of emitting copious secondary electrons when bombarded by electrons, a surrounding cylindrical metallic anode Il, and a pair of coils I'2, I2 for producing a magnetic field having ilux lines extending approximately parallel to the cathode. The anode, it should be noted, has end portions or closures I5 and these end portions are closer to the cathode than the cylindrical portion of the anode. The space between the anode and cathode is evauated in the manner of the customary vacuum tube or magnetron. This can be done by surrounding the anode with an air-tight glass envelope or, as shown, by making the anode as the envelope and sealing off the portions through which the leads extend by means of a glass seal I3. The metallic anode is apertured at i4 and the aperture `tightly sealedby a'transparent window i5 to permit light rays to shine on the cathode i0.

Fig. 1 shows one manner of impressing modulated light waves on the cathode. An illuminating light source I1, in the form of a lamp, throws light onto the soundtrack of .an .advancing motion picture film I8 through ya lens IB which focusses the light upon a mask, not shown, adjacent the film. Light passing through the sound track of the film is modulated and passes through the window I6 for shining-upon the cathode i0. The 'beam of light impinging upontheflm l is elongatedincross-section with the majoraxis of the elongated beam lying transverse of Athe beam.

Fig. 2 shows a .complete circuit arrangement for using the phototube secondary emissive ydevice'of Figs. l and la.. In Fig. 2 there is provided .a radio frequency parallel ltuned `oscillatory circuit .2G coupled across the anode tand 'cathodethrough a radio frequency by-pass condenser 2 l. A `power source2-2 of radio frequency current is magnetically coupled tothe tuned circuit 20, as shown. A direct current power source 23 supplies energizing current for the eld coil I2 over lleads `24, and supplies a suitable positive polarizing potential between cathode ID and vanode Il over a-path which includes the resistor 25, the primary .winding of the audio coupling transformer `26, and the-coil of tuned circuit '20. The audio frequency output modulations 4are amplified in audio amplifier 2l and then utilized in a suitable circuit such as a loudspeaker or headphones, not shown. The radio frequency bypass condenser 2i prevents the radio I)frequency energy in tuned circuit 2i? from entering the audio transformer 28.

A relatively long time constant'volume-control circuit comprising a vacuum tube 28 serves to maintain the average direct current through ythe phototube substantially constant,and this is done by varying a load on the tuned circuit 20 by means of coil 29 which is coupled to the tuned circuit "2% and connected between the anode and cathode of the tube .28., The control electrode or grid of tube 28 is connected to one terminal of resistor 25 whose other terminal Vis connected to the cathodethrough a resistor-condenser arrangement 30. A condenser 3| in shunt to resistor 25 Vby-passes all alternating currents and gives only a small response to currents of modulation frequency and higher. )In this way the phototube sensitivity is controlled, and limiting in the phototube amplifier is prevented.

. Abrief exposition of the principles underlying 4the 'operation of the'lpresent invention, and which 'is described'in my copending application, Serial No. 477,062, will now begiven. .If the anode-tocathode potential of the phototube is somewhat `greater than the cut-off potential corresponding to 'a particular intensity of magnetic field chosen, then nearly all :the electrons which leave the cathode 'l0 will strike the anode H on the rst outward trip. In other words, the direct current potential is adjusted high enough to prevent the accumulation of space charge. Now,^if the anode .potential is reduced rapidly, as by pulsing or the application of a superimposed alternating potential on the anode circuit, to a value below cutoff (i. e., below the value at which the electrons strike the anode on their first outward trip), then electrons will begin to miss the anode and return to the cathode. If the reduction or fall in anode potential is sufficiently rapid, electrons idrawnfrom the cathode or from the space close `to it, while the potential is falling, will circulate out from the cathode and return to the .cathode with an excess of energy, striking the cathode and causing the emission of additional electrons due to secondary emission. Thus, while the anode-to-cathode potential is decreasing, if the decrease is within a suitable range of rates, there isv obtained a rapid growth in total cathode emission due to secondary emission. Up to the point where space charge limiting becomes a factor, this increase in emission is an exponential function of time. The mammum emission ma ,in a very short time, reach values enormously greater than the photoemission but `still may have values proportional to the photoemission. If, before substantial space charge limiting sets in, the anode-to-cathode potential reaches low values then the emission will automatically stop rising and will decrease again to nearly Zero. Thus large emission takes place in pulses, repeated once each cycle of the radio frequency potential applied between anode and cathode. The value of the pulses of emission, both instantaneous and average, is proportional to the photoemission but lmay be enormously greater than the photoemission.

If the device is suitably designed, and peak emissions are allowed `to become large enough, magnetron oscillations may .take place which may add still further to the production of emission. When, due to the growth of total emission from secondary emission, the total space charge has become great enough, high frequency oscillation in the anode circuit will start, due to the condition of negative resistance. Oscillations will conto a Value too low to maintain oscillation. The

presence of oscillations adds to electron bombardment of the cathode in a manner to increase and prolong the production of secondary emission.

It has also been pointed out in my copending application that the same result, in producing growth `of electron emission, obtained by a rapidly decreasing electric eld can alternatively be achieved by a rising magnetic field. That is, an alternating magnetic field, in place of, Yor in addition to, the high frequency electric elcl may be used to cause the growth in electron emission from the cathode due to secondary emission.

Likewise, decreasing electric eld and increasing magnetic field may be used together to cause the growth of cathode emission due to secondary emission.

The phototube device of the present invention, as illustrated in Figs. 1 and la, and as used in Fig. 2, is so designed that electrons are pulled out of the space charge longitudinally, by components of motion parallel to the magnetic eld and caused to strike the lanode end portions i5. This is shown very roughly by the dotted lines of Fig. 1u.. Thus, during the growth of space charge, Aa portion of the electrons move oi the end of the ,cathode to the end closure i 5, even though at this particular time the anode-to-cathode Vpotential may be below cut-01T potential for electrons which would have to move at right angles to themagnetic field. It is only the components of `motion of the electrons which are at right angles to the ux lines which are bent or curved in direction to produce a rotating space charge. v As a consequence of the longitudinally moving electrons to the end portion l5, there is an ave-rage direct current flowing to the end closure l5 which may be modulated by light intensity. This modulated direct current, supplied from the direct current potential source 23 in series with the primary winding of transformer 26 constituting an output coupling impedance and in series with the magnetron phototube, then provides useful audio output power. This modulated direct current is enormously greater than the initial photoemission current.

In .the practice of the present invention, it is not essential that oscillations occur, although the growth of secondary emission will be aided by the onset of magnetron oscillations. This is because, during oscillation, the out-of-phase electrons strike the cathode with sufficient veloci-ty to cause secondary emission even when the anode potential is not falling rapidly enough to cause growth of emission by itself. An advantage of allowing oscillations is that it permits a reduction in the applied alternating anode-to-cathode potential.

To explain the operation in greater detail, assume that light causes emission of an electron from the cathode at a time when the anode-tocathode potential is somewhat less than the cutoff potential and is decreasing rapidly due to the high frequency excitation. Because of the electric field between anode and cathode, the electron will be accelerated away from the cathode toward the anode but the magnetic eld will bend its path and cause it to return to the cathode after an excursion out toward the anode. Because of the falling potential and falling electric eld, the electron is given more energy in moving away from the vcathode than is given to it while returning to the cathode. It therefore returns to the cathode with considerable energl7 and velocity and, if the energy is great enough, each returning electron can, on the average, cause the impact emission of more lthan one secondary electron. The maximum possible ratio of secondary emission electrons to impacting electrons, for optimum impact potentials, may vary from some very k low Value up to 10 or more, depending upon the character of the cathode surface. A ratio of 2 appears to be reasonable for surfaces capable of good life and stability. In practice, the surfaces used are unlikely to be uniform in manufacture and will vary with age but the variations may generally be compensated for by operating adjustments.

In using oxide cathodes, made by reducing barium and strontium carbonates to oxides in vacuum, which type of cathode is photoemissive, I have been able, with magnetrons, to demonstrate growth in emission current from cold cathodes, due to falling anode-to-cathode potential followed by radio frequency magnetron oscillation, from values too small to measure up to or amperes peak Value. It appears that a single electron, released from the cathode at the proper time, is suflicient to cause a growth to space charge limiting in the device of the invention, under suitable conditions of operation.

Assuming a ratio of secondary electrons to impacting electrons of 2, vthen the total cathode emission will be doubled in each time interval taken by electrons to make their excursion from the cathode out toward the anode and back. The rate or frequency of doubling is determined principauy by tne'strength' of the magnetic neld and.

F=2.29X (10) 6H where H is the magnetic field strength in gausses.

In practical magnetron phototubes, according to the invention, magnetic eld strengths up to say 2200 gausses are practical which would correspond to doubling the cathode emission at a rate of 5X (l0)9 times per second. Therefore, the time for each period of falling potential and growth of emission may be very short even when the multiplication of photoemission by secondary emission in each period is very large.

The foregoing formula has been given without considering the effect of accumulation of Space charge due to photoemission while the anode-tocathode potential is rising, assuming it does not rise all the way to the cut-olf potential for the strength of magnetic field used. If this is taken into account, the initial impacting current ,to start secondary emission, when the potentialbegins to decrease, may be greater than the current due to photoemission.

As for the operation of the volume control circuit of Fig. 2, as current flows in the anode l I the grid of vacuum tube 28 becomes more positive relative to the cathode of tube 28 as a result of which the effective resistance of tube 28 is lowered and additional loading is thrown on the radio frequency tuned circuit 20, thus tending to keep the direct current through the magnetron phototube substantially constant. Any other well known arrangement for controlling a high frequency potential in response to a direct current potential may be used. The arrangement is simply a slow acting automatic volume control to prevent overloading of the phototube amplifier tube.

Fig. 3 shows an alternative arrangement to that of Fig. 2. In Fig. 3, the arrangement of the anode lllA is different from that of I0 of Fig. 2, in that Fig. 3 employs an auxiliary electron collector output anode 32 to which are joined cooling fins 34. Auxiliary anode 32, in effect, serves the same purpose as the end closure I5 of Figs. 1 and 2 but may be operated at a lower potential to reduce power loss. -A separate glass envelope 33 serves to maintain an evacuated space within the phototube. The anode lil is apertured at I4 to permit modulated light to shine on the cathode. The radio frequency source 22 is coupled to tuned circuit 29 for varying the potential of anode to cathode at the radio frequency of source 22. The audio output transformer 26 is here shown coupled between the auxiliary electron collector anode 32 and the anode I Of course, means to provide the required magnetic field through the pliototube would be used with the device of Fig. 3, though it is not shown therein.

Fig. 4 illustrates .another vembodiment of the l invention which. uses an alternating current magnetic field insteadof a variable direct current f anode to cathode voltage for causing a growth of secondary emission. The two field coils l2, I2 are excited from an alternating current. power source 35 through field and generator tuning condensers 36, 36. The anode is shown in dotted.

lines, since in practice it will preferably be sectionalized to prevent short-circuiting eddy currents due to the alternating current magnetic field; The source of light is not shown in the interest of simplification of the drawings, and in practice would be impressed through the' glass Aenvelope and through the'aperture between the anode sectionsupon the cathode.

What is claimed is:-

1. An amplifier having within an envelope a photo-emissive cathode which emits secondary electrons due to electron bombardment at a ratio greater than unity, and an anode surrounding said cathode, said anode having an aperture and said envelope having a transparent portion registering with said aperture to enable light from an external source to strike said cathode, and means vto produce a periodically changing magnetic field having flux lines extending parallel to said cathode.`

2. An amplier having within an envelope a photo-emissive cathode which emits secondary electrons at a ratio greater than unity, and an anode surrounding said cathode, said anode having an aperture and said envelope having a transparent portionregistering with said aperture to enable light from an external source to strike said cathode, said anode having at least one closed end constituting an electron collecting surface in a plane at right angles to the axis of said cathode for collecting electrons which pass thereto by virtue of motions parallel to the magnetic iield, and means to produce a periodically changing electric field between said anode and cathode.

3. A photocell secondary emission ampliiier having within an evacuated envelope a photoemissive cathode which emits copious secondary electrons upon bombardment by primary electrons, an anode structure coaxial with and surrounding said cathode, said anode having an aperture and said envelope having a transparent portion registering with said aperture to enable light from an external source to strike said cathode, an electron collecting electrode with a surface adjacent one end of said cathode and more closely spaced relative to said cathode than said anode structure, said electron collecting electrode and said anode structure being directly connectedY together electrically, a source of unidirectional potential connected to said anode and said cathode for producing an electric field therebetween, means for producing a magnetic field having flux lines extending parallel to said cathode, and a source of alternating current coupled to said amplifier for periodically changing one of said fields.

4. An electron discharge device having within an evacuated envelope a photo-emissive cathode which emits secondary electrons at a ratio greater than unity, and an anode surrounding said cathode, said anode having an aperture and said envelope having a transparent portion registering with said aperture to enable light from an external source to strike said cathode, said anode having at least one closed end constituting an electron collecting electrode with a surface in a plane at right angles to the axis of said cathode for collecting electrons which pass thereto by virtue of motions parallel to the magnetic field, means for producing an electric field between said anode and cathode means for producing a magnetic field having flux lines extending parallel to said cathode, and at right angles to the magnetic eld, a source of alternating current potential coupled to said device for causing said electric lield to periodically change. at a rapid rate, through values less than the cut-oli" value for the selected intensity of magnetic field;` whereby photo-electrons are caused toy return to said cathode to produce secondary electrons during a portion of the cycle of said alternating current.

5, A phototube secondary emission amplifier having within an evacuated envelope a. photoemissive cathode which emits secondary electrons upon bombardment by primary electrons, an anode structure surrounding said cathode, said anode having an aperture and said envelope having a transparent portion vregistering with said aperture to enable light from an external source to strikesaid cathode, an electron collecting surface adjacent one end of said cathode andv more closely spaced relative to said cathode than said anode structure, said electron collecting surface and said anode structure being integral with one another,` a source of unidirectional potential connected between said anode and cathode for maintaining said anode and said electron collecting surface at a positive potential relative tov said cathode, a tuned radio frequency circuit in shunt to said anode and cathode, a source of radio frequency current coupled to said tuned circuit for varying the electric eld between said anode and cathode at a radio frequency rate, and means for producing a magnetic field having fluxk lines parallel to said cathode.

6. An amplifier having within an envelope a phGto-emissive cathode which emits secondary electrons at a ratio greater than unity, and an anode surrounding said cathode, said anode having an aperture and said envelope having a transparent portion registering with said aperture to enable light from an external source to .strike said cathode, said anode having at least one closed end adjacent one end of said cathode and constituting an electron collecting surface to collect electrons which pass thereto by virtue of motions parallel to the magnetic field, means to produce an electric eld between said anode and cathode, means to produce a magnetic field having flux lines extending parallel to said cathode, and a source of cyclically varying current coupled to said amplier for causing at least one of said fields to periodically change at a radio frequency rate,

whereby photo-electrons are caused to return to said cathode to produce secondary electrons.

7. A photocell secondary emission amplifier having within an evacuated envelope, a photoemissive cathode which emits copious secondary electrons upon bombardment by primary electrons, an anode structure surrounding said cathode, said anode having an aperture and said envelope having a transparent portion registering with said aperture to enable light from anexternal source to strike said cathode,- an electron collecting surface adjacent one end of said cathode and more closely spaced relative to said cathode than said anode structure, said electron collecting surface and said anode structure being integral with one another, a source of unidirecfield having flux lines parallel to said cathode, a

volume control circuit coupled to said tuned circuit and responsive to the average direct current through said amplier to maintain this current nearly constant.

8. An electron discharge device system cornprising a photo-emissive cathode and means in circuit therewith for causing the photo-electrons emitted by said cathode to return to and bombard said cathode to produce secondary electrons, said means comprising a source of unidirectional current for producing an electric field and a source of variable current for producing an increasing magnetic eld.

9. An electron discharge device system comprising a photo-emissive cathode and means in magnetic eld in the space between said cathode and anode with the lines of force substantially parallel to said cathode, and means coupled to said system and producing an electric eld between said anode and cathode, the values of said fields being such that photo-electrons emitted by said cathode return to and bombard said cathode to produce secondary electrons.

11. In an electron discharge device system, means for multiplying an electron current, produced by photo-emission from a cathode, comprising an anode, a magnetic field, combined direct and alternating high frequency electric elds at right angles to the magnetic field, means for collecting and utilizing a portion of the electrons emitted in the form of an anode to cathode current, and space discharge means in circuit with said system and responsive to the flow of electron current in said system for automatically controlling the average value of said anode to cathode current.

CLARENCE W. HANSELI... 

